GEO 101 Lecture Notes - Lecture 10: Chert, Gypsum, Jasper
23 Jun 2018
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Clastic Sedimentary Rocks
Clastic sedimentary rocks are classified according to the grain size of the sediment
and the kinds of rock fragments that make up the sediment (Table ). Grain size is
largely a function of the distance the particle was transported. In general, the greater the
distance traveled, the smaller and more rounded the sediment particles will be. This
smoothing of rock fragments during transportation is called rounding.
Large, coarse, angular pieces of sediment will be deposited near the source area; well‐
rounded sand grains will have been transported a considerable distance before being
deposited; silt, mud, and clay have been carried even farther. This process is
called sorting.
Coarse grained rocks. Sedimentary breccia‐contains an abundance of coarse,
angular fragments of gravel that were deposited very near the source area.
A conglomerate is formed from coarse grained, rounded pieces of ‐
gravel. Sandstone is a medium grained rock that contains rock particles (mostly quartz)‐
about the size of sand. The grains in a quartz sandstone are at least 90 percent
quartz. Graywacke is a type of “dirty” sandstone that consists of more than 15 percent
silt sized or clay sized (finer grained) material that imparts a darker or speckled ‐ ‐ ‐
appearance. If a coarse grained sandstone consists of over 25 percent feldspar grains it‐
is called an arkose.
Finer grained rocks.‐ The finer grained clastic sedimentary rocks are called shale, ‐
siltstone, and mudstone. Shale is a smooth, thinly layered rock that is made up of fine‐
grained silt and clay particles. Shale is considered a fissile rock because it splits very
naturally along its layers. A siltstone contains mostly silt grains and looks very similar
to shale but is not as fissile. Mudstone, the finest grained clastic rock, is not well ‐
layered, and contains more clay than does shale or siltstone. Most shales, siltstones,
and mudstones are tan, brown, gray, or black.
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Document Summary
Clastic sedimentary rocks are classified according to the grain size of the sediment and the kinds of rock fragments that make up the sediment (table ). Grain size is largely a function of the distance the particle was transported. In general, the greater the distance traveled, the smaller and more rounded the sediment particles will be. This smoothing of rock fragments during transportation is called rounding. Large, coarse, angular pieces of sediment will be deposited near the source area; well rounded sand grains will have been transported a considerable distance before being deposited; silt, mud, and clay have been carried even farther. This process is called sorting. contains an abundance of coarse, Sedimentary breccia angular fragments of gravel that were deposited very near the source area. A conglomerate is formed from coarse grained, rounded pieces of gravel. Sandstone is a medium grained rock that contains rock particles (mostly quartz) about the size of sand.
